Generally, in the case of laying heavyweight oil pipes, water pipes, and the like, under the ground, a dedicated pipe layer is used to carry the pipes to a place where the pipes are to be laid under the ground. In this case, the pipe layer can perform the basic operation (e.g. traveling, swing, and the like) of an excavator, and working devices (e.g. a boom, an arm, a bucket, and the like) are replaced by a boom, a winch, and the like.
The pipe layer may move to a place where oil pipes are to be laid under the ground or perform a swing operation in a state where a heavyweight oil pipe is pulled up using a flexible wire rope. In the case where the pipe layer travels or performs a swing operation at high speed or the traveling speed is abruptly increased or decreased, fatal problems may occur on the pipe layer.
Generally, according to the basic construction of a hydraulic circuit of an excavator, one of two hydraulic pumps drives left and right traveling motors, and the other thereof drives working devices such as a boom, an arm, and the like. In the case of driving the working device in a state where a traveling operation is performed, declination of the equipment may occur due to an unbalanced state of hydraulic fluid being supplied to the respective traveling motors.
Accordingly, in the case of driving the working device during traveling, a straight traveling valve is used to prevent the declination. That is, in the case where the traveling apparatus and the working device are simultaneously manipulated, one of the two hydraulic pumps takes complete charge of the hydraulic fluid being supplied to the left traveling motor and the right traveling motor through shifting of the straight traveling valve, and thus the declination of the equipment can be prevented.
In the case where only one of the working device and the traveling apparatus is driven in the shifting mode of the straight traveling valve (i.e. in the case where a combined work mode is released), the amount of hydraulic fluid being supplied to the traveling apparatus is changed, depending on the manipulation state of the working device, to cause an abrupt change of the traveling speed (i.e. increase or decrease of the traveling speed).
On the other hand, in the case of the excavator, a safety accident such as in the pipe layer may not occur due to the structural characteristic of the working devices, such as a boom, an arm, and the like, composed of rigid members even if the traveling speed is abruptly changed during traveling.
However, if the traveling speed is abruptly changed during traveling in a state where the pipe layer pulls up the oil pipe, the oil pipe shakes in a traveling direction of the equipment due to inertia, and severe safety accident may occur. In this case, the oil may collide with a part of the equipment neighboring the oil pipe or may secede from a wire rope and fall down.